Erasable programmable non-volatile memory

ABSTRACT

An erasable programmable non-volatile memory includes a first select transistor, a first floating gate transistor, a second select transistor and a second floating gate transistor. A select gate and a first source/drain terminal of the first select transistor receive a select gate voltage and a first source line voltage, respectively. A first source/drain terminal and a second source/drain terminal of the first floating gate transistor are connected with a second source/drain terminal of the first select transistor and a first bit line voltage, respectively. The second select transistor also includes the select gate. A first source/drain terminal of the second select transistor receive a second source line voltage. A first source/drain terminal and a second source/drain terminal of the second floating gate transistor are connected with the second source/drain terminal of the second select transistor and a second bit line voltage, respectively.

This application claims the benefit of U.S. provisional application Ser. No. 62/690,894, filed Jun. 27, 2018, the subject matter of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a non-volatile memory, and more particularly to an erasable programmable non-volatile memory.

BACKGROUND OF THE INVENTION

FIGS. 1A˜1D schematically illustrate the structure and the equivalent circuit of a conventional erasable programmable non-volatile memory. For example, the conventional erasable programmable non-volatile memory is disclosed in U.S. Pat. No. 8,941,167. FIG. 1A is a schematic top view of a conventional non-volatile memory. FIG. 1B is a schematic cross-sectional view illustrating the conventional non-volatile memory of FIG. 1A and taken along a first direction (a1-a2). FIG. 1C is a schematic cross-sectional view illustrating the non-volatile memory of FIG. 1A and taken along a second direction (b1-b2). FIG. 1D is a schematic equivalent circuit diagram of the conventional non-volatile memory.

As shown in FIGS. 1A and 1B, the conventional non-volatile memory comprises two serially-connected p-type transistors. These two p-type transistors are constructed in an n-type well region (NW). Three p-type doped regions 31, 32 and 33 are formed in the n-type well region (NW). In addition, two polysilicon gates 34 and 36 are spanned over the areas between the three p-type doped regions 31, 32 and 33.

The first p-type transistor is used as a select transistor, and the polysilicon gate 34 (also referred as a select gate) of the first p-type transistor is connected to a select gate voltage V_(SG). The p-type doped region 31 is connected to a source line voltage V_(SL). The p-type doped region 32 is a combination of a p-type doped drain region of the first p-type transistor and a p-type doped region of the second p-type transistor. The second p-type transistor is a floating gate transistor. The polysilicon gate 36 (also referred as a floating gate) is disposed over the second p-type transistor. The p-type doped region 33 is connected to a bit line voltage V_(BL). Moreover, the n-type well region (NW) is connected to an n-well voltage V_(NW).

As shown in FIGS. 1A and 1C, the conventional non-volatile memory further comprises an n-type transistor. The n-type transistor is composed of the floating gate 36 and an erase gate region 35. The n-type transistor is constructed in a p-type well region (PW). An n-type doped region 38 is formed in the p-type well region (PW). That is, the erase gate region 35 contains the p-type well region (PW) and the n-type doped region 38.

As shown in FIG. 1A, the floating gate 36 is extended externally and located near the erase gate region 35. Consequently, the floating gate 36 is also the gate terminal of the n-type transistor. Moreover, the n-type doped region 38 may be considered as a combination of an n-type doped source region and an n-type doped drain region. The n-type doped region 38 is connected to an erase line voltage V_(EL). In addition, the p-type well region (PW) is connected to a p-well voltage V_(PW). As shown in FIG. 1C, the erase gate region 35 and the n-type well region (NW) are isolated from each other by a shallow trench isolation (STI) structure 39.

As shown in the equivalent circuit of FIG. 1D, the non-volatile memory comprises a select transistor, a floating gate transistor and an n-type transistor. The select transistor and the floating gate transistor are p-type transistors and constructed in the n-type well region (NW). The n-type well region (NW) receives the n-well voltage V_(NW). The n-type transistor is constructed in the p-type well region (PW). In addition, the p-type well region (PW) receives the p-well voltage V_(PW).

The select gate of the select transistor receives the select gate voltage V_(SG). The first source/drain terminal of the select transistor receives the source line voltage V_(SL). The first source/drain terminal of the floating gate transistor is connected with the second source/drain terminal of the select transistor. The second source/drain terminal of the floating gate transistor receives the bit line voltage V_(BL). The gate terminal of the n-type transistor and the floating gate of the floating gate transistor are connected with each other. The first source/drain terminal of the n-type transistor and the second source/drain terminal of the n-type transistor are connected with each other to receive the erase line voltage V_(EL).

SUMMARY OF THE INVENTION

The present invention provides an erasable programmable non-volatile memory with a novel structure.

An embodiment of the present invention provides an erasable programmable non-volatile memory. The erasable programmable non-volatile memory includes a first select transistor, a first floating gate transistor, a second select transistor and a second floating gate transistor. The first select transistor includes a select gate, a first source/drain terminal and a second source/drain terminal. The select gate of the first select transistor receives a select gate voltage. The first source/drain terminal of the first select transistor receives a first source line voltage. The first floating gate transistor includes a floating gate, a first source/drain terminal and a second source/drain terminal. The first source/drain terminal of the first floating gate transistor is connected with the second source/drain terminal of the first select transistor. The second source/drain terminal of the first floating gate transistor receives a first bit line voltage. The second select transistor includes the select gate, a first source/drain terminal and a second source/drain terminal. The first source/drain terminal of the second select transistor receives a second source line voltage. The second floating gate transistor includes the floating gate, a first source/drain terminal and a second source/drain terminal. The first source/drain terminal of the second floating gate transistor is connected with the second source/drain terminal of the second select transistor. The second source/drain terminal of the second floating gate transistor receives a second bit line voltage. The first select transistor and the first floating gate transistor are constructed in a first well region, the second select transistor and the second floating gate transistor are constructed in a second well region. The first well region and the second well region have different types.

Another embodiment of the present invention provides an erasable programmable non-volatile memory. The erasable programmable non-volatile memory includes a semiconductor layer, a first well region, a first doped region, a second doped region, a third doped region, a second well region, a fourth doped region, a fifth doped region, a sixth doped region, a select gate, a floating gate and an isolation structure. The first well region is formed in the semiconductor layer. The first doped region, the second doped region and the third doped region are formed in a surface of the first well region. The first doped region receives a first source line voltage. The third doped region receives a first bit line voltage. The second well region is formed in the semiconductor layer. The fourth doped region, the fifth doped region and the sixth doped region are formed in a surface of the second well region. The fourth doped region receives a second source line voltage. The sixth doped region receives a second bit line voltage. The select gate is spanned over an area between the first doped region and the second doped region and an area between the fourth doped region and the fifth doped region. The select gate receives a select gate voltage. The floating gate is spanned over an area between the second doped region and the third doped region and an area between the fifth doped region and the sixth doped region. The isolation structure is formed in the semiconductor layer and arranged between the first well region and the second well region.

Numerous objects, features and advantages of the present invention will be readily apparent upon a reading of the following detailed description of embodiments of the present invention when taken in conjunction with the accompanying drawings. However, the drawings employed herein are for the purpose of descriptions and should not be regarded as limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which:

FIGS. 1A˜1D (prior art) schematically illustrate the structure and the equivalent circuit of a conventional erasable programmable non-volatile memory;

FIG. 2A is a schematic top view illustrating an erasable programmable non-volatile memory according to a first embodiment of the present invention;

FIG. 2B is a schematic cross-sectional view illustrating the non-volatile memory as shown in FIG. 2A and taken along the dotted line a-b;

FIG. 2C is a schematic equivalent circuit diagram of the erasable programmable non-volatile memory of FIG. 2A;

FIGS. 3A˜3D schematically illustrate the bias voltages and the operations of the erasable programmable non-volatile memory according to the first embodiment of the present invention;

FIG. 4A is a schematic top view illustrating an erasable programmable non-volatile memory according to a second embodiment of the present invention; and

FIG. 4B is a schematic cross-sectional view illustrating the non-volatile memory as shown in FIG. 4A and taken along the dotted line c-d.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 2A is a schematic top view illustrating an erasable programmable non-volatile memory according to a first embodiment of the present invention. FIG. 2B is a schematic cross-sectional view illustrating the non-volatile memory as shown in FIG. 2A and taken along the dotted line a-b. FIG. 2C is a schematic equivalent circuit diagram of the erasable programmable non-volatile memory of FIG. 2A.

As shown in FIGS. 2A and 2B, the erasable programmable non-volatile memory 20 comprises two serially-connected p-type transistors and two serially-connected n-type transistors. These two p-type transistors are constructed in an n-type well region (NW). The two serially-connected n-type transistors are constructed in a p-type well region (PW). The n-type well region (NW) and the p-type well region (PW) are separated from each other by an isolation structure 29. For example, the isolation structure 29 is a shallow trench isolation (STI) structure. In this embodiment, the isolation structure 29, the n-type well region (NW) and the p-type well region (PW) are formed in a p-type substrate (p_sub). Alternatively, the isolation structure 29, the n-type well region (NW) and the p-type well region (PW) are formed in another semiconductor layer such as an n-type substrate, an n-type buried layer (NBL) or a deep n-well (DNW) region.

Three p-type doped regions 21, 22 and 23 are formed in the n-type well region (NW). In addition, a polysilicon gate SG is spanned over the area between the p-type doped regions 21 and 22, and a polysilicon gate FG is spanned over the area between the p-type doped regions 22 and 23. The first p-type transistor is used as a first select transistor, and the polysilicon gate SG (also referred as a select gate) of the first p-type transistor is connected to a select gate voltage V_(SG). The p-type doped region 21 is connected to a first source line voltage V_(SL1). The p-type doped region 22 is a combination of a p-type doped drain region of the first p-type transistor and a p-type doped region of the second p-type transistor. The second p-type transistor is used as a first floating gate transistor. The polysilicon gate FG (also referred as a floating gate) is disposed over the second p-type transistor. The p-type doped region 23 is connected to a first bit line voltage V_(BL1). Moreover, the n-type well region (NW) is connected to an n-well voltage V_(NW).

Three n-type doped regions 25, 26 and 27 are formed in the p-type well region (PW). In addition, two polysilicon gates SG and FG are spanned over the areas between the three n-type doped regions 25, 26 and 27. The first n-type transistor is used as a second select transistor, and the polysilicon gate SG (also referred as a select gate) of the second n-type transistor is connected to the select gate voltage V_(SG). The n-type doped region 25 is connected to a second source line voltage V_(SL2). The n-type doped region 26 is a combination of an n-type doped drain region of the first n-type transistor and an n-type doped region of the second n-type transistor. The second n-type transistor is used as a second floating gate transistor. The polysilicon gate FG (i.e., the floating gate) is disposed over the second n-type transistor. The n-type doped region 27 is connected to a second bit line voltage V_(BL2). Moreover, the p-type well region (PW) is connected to a p-well voltage V_(PW).

In this embodiment, the select gate SG of the first select transistor and the select gate SG of the second select transistor are formed of the same polysilicon gate. That is, the select gate SG of the first select transistor and the select gate SG of the second select transistor are connected with each other. Moreover, the floating gate FG of the first floating gate transistor and the floating gate FG of the second floating gate transistor are formed of the same polysilicon gate. That is, the floating gate FG of the first floating gate transistor and the floating gate FG of the second floating gate transistor are connected with each other. Moreover, the channel length L1 of the first floating gate transistor is smaller than the channel length L2 of the second floating gate transistor, and the channel width w1 of the first floating gate transistor is smaller than the channel width w2 of the second floating gate transistor.

As shown in the equivalent circuit of FIG. 2C, the erasable programmable non-volatile memory 20 comprises a first select transistor, a second select transistor, a first floating gate transistor and a second floating gate transistor. That is, the four transistors constitute a memory cell of the erasable programmable non-volatile memory 20. The memory cell is connected with two bit lines, two source lines and two select lines connected with each other (i.e., the select gate SG of the first select transistor and the select gate SG of the second select transistor are connected with each other).

The first select transistor and the first floating gate transistor are p-type transistors and constructed in the n-type well region (NW). The n-type well region (NW) receives the n-well voltage V_(NW). The second select transistor and the second floating gate transistor are n-type transistors and constructed in the p-type well region (PW). In addition, the p-type well region (PW) receives the p-well voltage V_(PW).

The select gate SG of the first select transistor receives the select gate voltage V_(SG). The first source/drain terminal of the first select transistor receives the first source line voltage V_(SL1). The first source/drain terminal of the first floating gate transistor is connected with the second source/drain terminal of the first select transistor. The second source/drain terminal of the first floating gate transistor receives the first bit line voltage V_(BL1).

The select gate SG of the second select transistor receives the select gate voltage V_(SG). The first source/drain terminal of the second select transistor receives the second source line voltage V_(SL2). The first source/drain terminal of the second floating gate transistor is connected with the second source/drain terminal of the second select transistor. The second source/drain terminal of the second floating gate transistor receives the second bit line voltage V_(BL2).

The operations of the erasable programmable non-volatile memory 20 will be described as follows.

During a program cycle of the non-volatile memory 20, the first select transistor and the first floating gate transistor are activated. Consequently, hot carriers (e.g. electrons) are transferred through a channel region of the first floating gate transistor and injected into the floating gate FG.

During an erase cycle of the non-volatile memory 20, the first select transistor and the first floating gate transistor are activated. Consequently, the electrons are ejected from the floating gate FG through the channel region of the first floating gate transistor.

In a read cycle of the non-volatile memory 20, the second select transistor and the second floating gate transistor are activated. According to the amount of electrons in the floating gate FG, the second floating gate transistor generates a read current. According to the magnitude of the read current, the storage state of the non-volatile memory 20 is determined.

FIGS. 3A˜3D schematically illustrate the bias voltages and the operations of the erasable programmable non-volatile memory according to the first embodiment of the present invention.

As shown in FIGS. 3A and 3B, the program cycle is divided into two time periods. For example, the program cycle needs 50 μs, the first time period (or an earlier stage) of the program cycle is 20 μs, and the second time period (or a later stage) of the program cycle is 30 μs.

In the earlier stage of the program cycle, the select gate voltage V_(SG), the second source line voltage V_(SL2), the second bit line voltage V_(BL2) and the p-well voltage V_(PW) are all 0V. Consequently, the second select transistor is turned off. Moreover, the select gate voltage V_(SG) is 0V, the first source line voltage V_(SL1) is 5V, the first bit line voltage V_(BL1) is −2V, and the n-well voltage V_(NW) is 5V. In this situation, the first select transistor is turned on, and a program current Ipgm flows through the first select transistor. Consequently, electrons are injected into the floating gate FG through the channel region of the first floating gate transistor.

In the later stage of the program cycle, the second source line voltage V_(SL2), the second bit line voltage V_(BL2) and the p-well voltage V_(PW) are all increased to 5V while the select gate voltage V_(SG) is 0V. Consequently, the floating gate FG of the second floating gate transistor is coupled to have the voltage of about 5V. Under this circumstance, more electrons are injected into the floating gate FG of the first floating gate transistor. Consequently, the program efficiency of the non-volatile memory 20 is enhanced.

Please refer to FIGS. 3A and 3C. During the erase cycle, the second source line voltage V_(SL2), the second bit line voltage V_(BL2) and the p-well voltage V_(PW) are all −7.5V. In addition, the first source line voltage V_(SL1), the first bit line voltage V_(BL1) and the n-well voltage V_(NW) are all 7.5V while the select gate voltage V_(SG) is 0V. Consequently, the first select transistor and the second select transistor are turned off.

Since the floating gate FG of the second floating gate transistor is coupled to have the voltage of about −7.5V and both of the first bit line voltage V_(BL1) and the n-well voltage V_(NW) are 7.5V, a voltage difference between the floating gate FG of the first floating gate transistor and the n-well voltage V_(NW) is 15V. Consequently, the electrons are ejected from the floating gate FG of the first floating gate transistor to the n-type well region (NW).

Please refer to FIGS. 3A and 3D. During the read cycle, the select gate voltage V_(SG), the first source line voltage V_(SL1), the first bit line voltage V_(BL1) and the n-well voltage V_(NW) are all 2.5V. Consequently, the first select transistor is turned off. In addition, the select gate voltage V_(SG) is 2.5V, the second source line voltage V_(SL2) is 0V, the second bit line voltage V_(BL2) is 2.5V, and the p-well voltage V_(PW) is 0V. Consequently, the second select transistor is turned on, and the second floating gate transistor generates a read current Iread.

Generally, the magnitude of the read current Iread is determined according to the amount of electrons stored in the floating gate FG of the second floating gate transistor. For example, in case that electrons are stored in the floating gate FG of the second floating gate transistor, the second floating gate transistor is turned off and the magnitude of the generated read current Iread is very low (e.g., nearly zero). Whereas, in case that no electrons are stored in the floating gate FG of the second floating gate transistor, the second floating gate transistor is turned on and the magnitude of the generated read current Iread is higher. Consequently, the storage state of the non-volatile memory 20 is determined according to the magnitude of the read current Iread.

As mentioned above, the channel length L1 of the first floating gate transistor is smaller than the channel length L2 of the second floating gate transistor, and the channel width w1 of the first floating gate transistor is smaller than the channel width w2 of the second floating gate transistor. Consequently, the program efficiency of the first floating gate transistor is enhanced, and the read efficiency of the second floating gate transistor is enhanced.

Generally, the n-type well region (NW) and the p-type well region (PW) of the non-volatile memory 20 have high doping concentrations. During the erase cycle of the non-volatile memory 20, the n-well voltage V_(NW) is 7.5V and the p-well voltage V_(PW) is −7.5V. That is, the voltage difference between the n-type well region (NW) and the p-type well region (PW) is 15V. If the n-type well region (NW) and the p-type well region (PW) of the non-volatile memory 20 are in contact with each other, the junction breakdown voltage is not enough for sustaining during the erase action. Under this circumstance, a junction breakdown problem occurs.

As shown in FIG. 2B, a wider isolation structure 29 is formed in the p-type substrate (p_sub) to separate the n-type well region (NW) from the p-type well region (PW). For example, the width of the isolation structure 29 is larger than 1 μm. Because of the wider isolation structure 29, after the n-type well region (NW) and the p-type well region (PW) are produced, the n-type well region (NW) and the p-type well region (PW) are not in contact with each other and separated from each other by the p-type substrate (p_sub). Consequently, the junction breakdown problem of the non-volatile memory 20 is avoided.

Since the doping concentration of the p-type substrate (p_sub) is low, the junction breakdown voltage between the n-type well region (NW) and the p-type substrate (p_sub) is higher than conventional one. Consequently, during the erase cycle of the non-volatile memory 20, the junction breakdown problem is avoided. However, since the wider isolation structure 29 is formed in the p-type substrate (p_sub), the memory cell of the non-volatile memory 20 has a larger area.

FIG. 4A is a schematic top view illustrating an erasable programmable non-volatile memory according to a second embodiment of the present invention. FIG. 4B is a schematic cross-sectional view illustrating the non-volatile memory as shown in FIG. 4A and taken along the dotted line c-d. The bias voltages and the equivalent circuit of the non-volatile memory of this embodiment are similar to those of the first embodiment, and are not redundantly described herein.

In comparison with the first embodiment, the structures of the well regions of the second embodiment are distinguished. In this embodiment, the n-type well region comprises plural n-type sub-well regions, and the p-type well region comprises plural p-type sub-well regions. For example, the n-type well region comprises three n-type sub-well regions NW1˜NW3, and the p-type well region comprises three p-type sub-well regions PW1˜PW3. In other embodiments, the n-type well region may comprise two n-type sub-well regions or more than three n-type sub-well regions, and the p-type well region may comprise two p-type sub-well regions or more than three p-type sub-well regions.

As shown in FIGS. 4A and 4B, the erasable programmable non-volatile memory 40 comprises two serially-connected p-type transistors and two serially-connected n-type transistors. These two p-type transistors are constructed in the first n-type sub-well region (NW1). The two serially-connected n-type transistors are constructed in a first p-type sub-well region (PW1). The first n-type sub-well region (NW1) and the first p-type sub-well region (PW1) are separated from each other by an isolation structure 49. The second n-type sub-well region (NW2) and the second p-type sub-well region (PW2) are also separated from each other by the isolation structure 49. The third n-type sub-well region (NW3) and the third p-type sub-well region (PW3) are in direct contact with each other. For example, the isolation structure 49 is a shallow trench isolation (STI) structure. In this embodiment, the isolation structure 49, the n-type well region and the p-type well region are formed in a p-type substrate (p_sub). Alternatively, the isolation structure 49, the n-type well region and the p-type well region are formed in another semiconductor layer such as an n-type substrate, an n-type buried layer (NBL) or a deep n-well (DNW) region.

Three p-type doped regions 41, 42 and 43 are formed in the first n-type sub-well region (NW1). In addition, a polysilicon gate SG is spanned over the area between the p-type doped regions 41 and 42, and a polysilicon gate FG is spanned over the area between the p-type doped regions 42 and 43. The first p-type transistor is used as a first select transistor, and the polysilicon gate SG (also referred as a select gate) of the first p-type transistor is connected to a select gate voltage V_(SG). The p-type doped region 41 is connected to a first source line voltage V_(SL1). The p-type doped region 42 is a combination of a p-type doped drain region of the first p-type transistor and a p-type doped region of the second p-type transistor. The second p-type transistor is used as a first floating gate transistor. The polysilicon gate FG (also referred as a floating gate) is disposed over the second p-type transistor. The p-type doped region 43 is connected to a first bit line voltage V_(BL1). Moreover, the n-type well region is connected to an n-well voltage V_(NW).

Three n-type doped regions 45, 46 and 47 are formed in the first p-type sub-well region (PW1). In addition, two polysilicon gates SG and FG are spanned over the areas between the three n-type doped regions 45, 46 and 47. The first n-type transistor is used as a second select transistor, and the polysilicon gate SG (also referred as a select gate) of the second n-type transistor is connected to the select gate voltage V_(SG). The n-type doped region 45 is connected to a second source line voltage V_(SL2). The n-type doped region 46 is a combination of an n-type doped drain region of the first n-type transistor and an n-type doped region of the second n-type transistor. The second n-type transistor is used as a second floating gate transistor. The polysilicon gate FG (i.e., the floating gate) is disposed over the second n-type transistor. The n-type doped region 47 is connected to a second bit line voltage V_(BL2). Moreover, the p-type well region is connected to a p-well voltage V_(PW).

During the formation of the well regions, plural ion implantation steps are performed according to different depths. Consequently, the plural n-type sub-well regions and the plural p-type sub-well regions are formed.

Please refer to FIG. 4B. From the shallower to the deeper, the n-type well region in the p-type substrate (p_sub) comprises the first n-type sub-well region (NW1), the second n-type sub-well region (NW2) and the third n-type sub-well region (NW3) sequentially. Similarly, from the shallower to the deeper, the n-type well region in the p-type substrate (p_sub) comprises the first p-type sub-well region (PW1), the second p-type sub-well region (PW2) and the third p-type sub-well region (PW3) sequentially. The doping concentration of the second n-type sub-well region (NW2) is higher than the doping concentration of the third n-type sub-well region (NW3). The doping concentration of the second p-type sub-well region (PW2) is higher than the doping concentration of the third p-type sub-well region (PW3).

Since the doping concentration of the third n-type sub-well region (NW3) and the doping concentration of the third p-type sub-well region (PW3) are low, the junction breakdown voltage between the third n-type sub-well region (NW3) and the third p-type sub-well region (PW3) is very high. Consequently, during the erase cycle, the junction breakdown problem of the non-volatile memory 40 is not generated.

Since the non-volatile memory 40 is not suffered from the junction breakdown problem, the isolation structure 49 in the p-type substrate (p_sub) may be narrower. For example, the width of the isolation structure 49 is smaller than 0.5 μm. After the n-type well region and the p-type well region are formed, the third n-type sub-well region (NW3) and the third p-type sub-well region (PW3) are in direct contact with each other and the junction breakdown problem of the non-volatile memory 40 is avoided.

As mentioned above, the doping concentration of the second n-type sub-well region (NW2) is higher than the doping concentration of the third n-type sub-well region (NW3), and the doping concentration of the second p-type sub-well region (PW2) is higher than the doping concentration of the third p-type sub-well region (PW3). Consequently, the punch through effect between the third n-type sub-well region (NW3) to n-type doped regions 45, 46, and 47 is not generated during the erase cycle.

Since the isolation structure 49 of the non-volatile memory 40 is narrower, the area of the memory cell can be effectively reduced.

Moreover, the threshold voltage of the first select transistor is determined according to the doping concentration of the first n-type sub-well region (NW1), and the threshold voltage of the second select transistor is determined according to the doping concentration of the first p-type sub-well region (PW1). Consequently, the doping concentration of the first n-type sub-well region (NW1) and the doping concentration of the first p-type sub-well region (PW1) are not restricted. The doping concentration of the first n-type sub-well region (NW1) and the doping concentration of the first p-type sub-well region (PW1) may be determined according to the practical requirements.

In the above embodiments, the p-type select transistor and the p-type floating gate transistor are responsible for the program action and the erase action, and the n-type select transistor and the n-type floating gate transistor are responsible for the read action. It is noted that numerous modifications and alterations may be made while retaining the teachings of the invention. For example, in another embodiment, the n-type select transistor and the n-type floating gate transistor constructed in the p-type well region are responsible for the program action and the erase action, and the p-type select transistor and the p-type floating gate transistor constructed in the n-type well region are responsible for the read action.

While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures. 

What is claimed is:
 1. An erasable programmable non-volatile memory, comprising: a first select transistor comprising a select gate, a first source/drain terminal and a second source/drain terminal, wherein the select gate of the first select transistor receives a select gate voltage, and the first source/drain terminal of the first select transistor receives a first source line voltage; a first floating gate transistor comprising a floating gate, a first source/drain terminal and a second source/drain terminal, wherein the first source/drain terminal of the first floating gate transistor is connected with the second source/drain terminal of the first select transistor, and the second source/drain terminal of the first floating gate transistor receives a first bit line voltage; a second select transistor comprising the select gate, a first source/drain terminal and a second source/drain terminal, wherein the first source/drain terminal of the second select transistor receives a second source line voltage; and a second floating gate transistor comprising the floating gate, a first source/drain terminal and a second source/drain terminal, wherein the first source/drain terminal of the second floating gate transistor is connected with the second source/drain terminal of the second select transistor, and the second source/drain terminal of the second floating gate transistor receives a second bit line voltage, wherein the first select transistor and the first floating gate transistor are constructed in a first well region, the second select transistor and the second floating gate transistor are constructed in a second well region, and the first well region and the second well region have different types.
 2. The erasable programmable non-volatile memory as claimed in claim 1, wherein the first select transistor and the first floating gate transistor are n-type transistors, and the first well region is a p-type well region that receives a p-well voltage, wherein the second select transistor and the second floating gate transistor are p-type transistors, and the second well region is an n-type well region that receives an n-well voltage.
 3. The erasable programmable non-volatile memory as claimed in claim 1, wherein the first select transistor and the first floating gate transistor are p-type transistors, and the first well region is an n-type well region that receives an n-well voltage, wherein the second select transistor and the second floating gate transistor are n-type transistors, and the second well region is a p-type well region that receives a p-well voltage.
 4. The erasable programmable non-volatile memory as claimed in claim 1, wherein the first floating gate transistor has a first channel length, and the second floating gate transistor has a second channel length, wherein the first channel length is smaller than the second channel length.
 5. The erasable programmable non-volatile memory as claimed in claim 1, wherein the first floating gate transistor has a first channel width, and the second floating gate transistor has a second channel width, wherein the first channel width is smaller than the second channel width.
 6. The erasable programmable non-volatile memory as claimed in claim 1, wherein during a program cycle, plural electrons are injected into the floating gate of the first floating gate transistor through a channel region of the first floating gate transistor.
 7. The erasable programmable non-volatile memory as claimed in claim 1, wherein during an erase cycle, plural electrons are ejected from the floating gate of the first floating gate transistor to the first well region.
 8. The erasable programmable non-volatile memory as claimed in claim 1, wherein during a read cycle, the second floating gate transistor generates a read current.
 9. The erasable programmable non-volatile memory as claimed in claim 8, wherein when plural electrons are stored in the floating gate of the second floating gate transistor, the second floating gate transistor generates a first read current, wherein when no electrons are stored in the floating gate of the second floating gate transistor, the second floating gate transistor generates a second read current, wherein the second read current is different from the first read current.
 10. An erasable programmable non-volatile memory, comprising: a semiconductor layer; a first well region formed in the semiconductor layer; a first doped region, a second doped region and a third doped region, which are formed in a surface of the first well region, wherein the first doped region receives a first source line voltage, and the third doped region receives a first bit line voltage; a second well region formed in the semiconductor layer; a fourth doped region, a fifth doped region and a sixth doped region, which are formed in a surface of the second well region, wherein the fourth doped region receives a second source line voltage, and the sixth doped region receives a second bit line voltage; a select gate spanned over an area between the first doped region and the second doped region and an area between the fourth doped region and the fifth doped region, wherein the select gate receives a select gate voltage; a floating gate spanned over an area between the second doped region and the third doped region and an area between the fifth doped region and the sixth doped region; and an isolation structure formed in the semiconductor layer and arranged between the first well region and the second well region.
 11. The erasable programmable non-volatile memory as claimed in claim 10, wherein the first well region is a p-type well region that receives a p-well voltage, and the second well region is an n-type well region that receives an n-well voltage, wherein the first doped region, the second doped region and the third doped region are n-type doped regions, and the fourth doped region, the fifth doped region and the sixth doped region are p-type doped regions.
 12. The erasable programmable non-volatile memory as claimed in claim 10, wherein the first well region is an n-type well region that receives an n-well voltage, and the second well region is a p-type well region that receives a p-well voltage, wherein the first doped region, the second doped region and the third doped region are p-type doped regions, and the fourth doped region, the fifth doped region and the sixth doped region are n-type doped regions.
 13. The erasable programmable non-volatile memory as claimed in claim 10, wherein the first well region, the second doped region, the third doped region and the floating gate are collaboratively formed as a first floating gate transistor, and the second well region, the fifth doped region, the sixth doped region and the floating gate are collaboratively formed as a second floating gate transistor.
 14. The erasable programmable non-volatile memory as claimed in claim 13, wherein the first floating gate transistor has a first channel length, and the second floating gate transistor has a second channel length, wherein the first channel length is smaller than the second channel length.
 15. The erasable programmable non-volatile memory as claimed in claim 13, wherein the first floating gate transistor has a first channel width, and the second floating gate transistor has a second channel width, wherein the first channel width is smaller than the second channel width.
 16. The erasable programmable non-volatile memory as claimed in claim 10, wherein the first well region comprises plural first-type sub-well regions, and the plural first-type sub-well regions are sequentially formed in the semiconductor layer from a surface of the semiconductor layer, wherein the first doped region, the second doped region and the third doped region are formed in a surface of a first one of the first-type sub-well regions of the first well region, wherein the second well region comprises plural second-type sub-well regions, and the plural second-type sub-well regions are sequentially formed in the semiconductor layer from the surface of the semiconductor layer, wherein the fourth doped region, the fifth doped region and the sixth doped region are formed in a surface of a first one of the second-type sub-well regions of the second well region.
 17. The erasable programmable non-volatile memory as claimed in claim 16, wherein a last one of the first-type sub-well regions of the first well region and a last one of the second-type sub-well regions of the second well region are in contact with each other.
 18. The erasable programmable non-volatile memory as claimed in claim 17, wherein the last one of the first-type sub-well regions of the first well region has a first doping concentration, and a second last one of the first-type sub-well regions of the first well region has a second doping concentration, wherein the second doping concentration is higher than the first doping concentration.
 19. The erasable programmable non-volatile memory as claimed in claim 18, wherein the last one of the second-type sub-well regions of the second well region has a third doping concentration, and a second last one of the second-type sub-well regions of the second well region has a fourth doping concentration, wherein the fourth doping concentration is higher than the third doping concentration.
 20. The erasable programmable non-volatile memory as claimed in claim 10, wherein the isolation structure is a shallow trench isolation structure.
 21. The erasable programmable non-volatile memory as claimed in claim 10, wherein the semiconductor layer is a p-type substrate, an n-type substrate, an n-type buried layer or a deep n-type well region. 